Electrolytic capacitors comprise an anode, which is made of a valve metal such as aluminum, tantalum or niobium and on the surface of which a dielectric or insulating oxide film layer is formed by anodization or some other means, and a cathode which faces the anode via a separator impregnated with a liquid electrolyte. The liquid electrolyte is in contact with the essentially dielectric oxide film layer and serves as the true cathode. Therefore, the characteristics such as the specific resistance of the liquid electrolyte have direct effects on the electrical characteristics of the electrolytic capacitor, and it is indispensable to employ a liquid electrolyte having good characteristics for the purpose of fabricating an electrolytic capacitor of high performance.
With a view to making liquid electrolytes having good characteristics, the use of tetraalkylammonium salts of dicarboxylic acids dissolved in aprotic polar solvents has recently been reviewed. Tetramethylammonium salts of dicarboxylic acids have drawn researchers' attention as electrolytes having low specific-resistance characteristics, and these salts are easier to synthesize than other tetraalkylammonium salts. However, because of their low solubility in aprotic polar solvents, the liquid electrolyte systems using the tetramethylammonium salts of dicarboxylic acids cannot be rendered satisfactorily low in specific resistance and are unable to ensure good characteristics at low temperatures. These problems have prevented the salts from exhibiting their inherent characteristics as electrolytes to the fullest extent.
A prior art proposal for using tetramethylammonium salts of dicarboxylic acids as electrolytes is shown in Unexamined Published Japanese Patent Application No. 78522/1984. In this prior patent, the specific resistance is reduced (electroconductivity improved) and the efficiency of electrochemical conversion is improved by both adding a small amount of water to aprotic polar solvents and introducing more cations in the electrolyte than anions in terms of equivalent ratio. However, the presence of water causes pressure buildup in the interior of the electrolyte capacitor when it is used at temperatures in excess of 100.degree. C., and this can lead to a shorter life of the capacitor. The use of water can also result in deterioration of the capacitor's characteristics at low temperatures.